[Dean]

The pivoting kind will allow the two sides of the car to displace relative to each other like a rhombus:

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Whereas one without a pivot will resist that to at least some extent.

Everything has some sort of flex, and preloading in the approrite direction is an attempt to minimize it.

Lets assume the firewalls are perfectly stiff, and pretend that the whole interior unibody and body fenders act as a single surface called the side panels.

The flex we are trying to reduce or at least control is what is allowed by the side panels under load from the strut top relative to the fixed point/line that is the firewall side panel intersection. by placing a rigid memeber between the two strut tops, we force any load applied to one side to have to work against the stiffness of it's side panel, as well as the other side's though the bar. By slightly preloading the bar in compression, we ensure there is no slop in the bar itself, and minimal defelction is allowed on one side before the other side's resistance comes into play.

In a small number of cases, you might preload the bar in tension to reduce body flex and possibly allow for additional negative camber.